There is a scene in episode three of marvel moon knight with a unique time travel. Oscar Isaac’s character Marc Spector stands under a clear night sky in Cairo, Egypt, where the band of the Milky Way shimmers purple and blue. With the help of the god Khonshu, they rewind the night sky to see what it looked like 2,000 years ago. Once the magic begins, the stars scroll clockwise until they finally stop at the ancient sky they were looking for.
In the Disney+ show, it’s magic – but the European Space Agency has an ambitious mission that can help scientists fast forward and rewind the sky, at least on computers.
This scene got a lot of things right, minus the Egyptian deities. The night sky changes from year to year, as a particular space observatory can prove. The European Space Agency’s (ESA) Gaia mission uses stellar motion and high-precision optics to create a three-dimensional map of the Milky Way to show how stars travel through time.
How does Gaia work?
In 2013, ESA sent the Gaia spacecraft to Lagrange Point 2 (L2), a gravitationally stable point in space located behind the Earth in relation to the sun, nearly a million kilometers from our planet. Gaia has shared this location with NASA’s James Webb Space Telescope since January 24.
Gaia is far enough away to get a clear view of the Milky Way without Earth interfering, but it’s close enough to communicate with scientists while accompanying Earth on its annual trip around the sun.
As part of the mission, Gaia scientists want to know what the sky looked like in the past and what it will look like in the future. Collisions of ancient galaxies and the eventual merger of the Milky Way with the Andromeda Galaxy are events that occur on time scales of billions of years. But Gaia’s particular finesse can see incremental changes over much shorter periods of time, leading to a more detailed understanding of how stars move. With a few years of position and motion data collection, this can help piece together the past and future movements of our celestial neighbors.
The first piece to solving this puzzle is taking into account some funny tricks of perception.
Gaia uses parallax, this interesting phenomenon where you close one eye, stare at an object, then close it and open the other eye, and the object appears to have moved.
One way to look at Gaia’s technique is to pretend that the Sun is your nose and Earth’s orbit stretches between your eyes. If you only have your right eye open, you will see the objects in your room – the surroundings of the Milky Way – as they appear in summer. Close that one and open the left eye, and this is what they’ll look like six months later from Earth’s new place in space. Gaia studies the sky in periods of 180 days, or about six Earth months. So Gaia looks up at the sky when she’s on one side of the Sun, then again when she’s on the other side.
Each period, the stars of the Milky Way will always appear in a very slightly different place compared to incredibly distant light beacons like quasars that do not move so much. “Quasars are very distant objects outside our Milky Way and they have proper motion which is zero because Gaia is not able to measure it. So if we see proper motions, that means it’s due to the acceleration of the Sun”, explains Antonella Vallenari, scientist collaborating at Gaia. Reverse.
Stars closer to Earth will appear to move more than those farther away, revealing their distances. Moon Knight’s rewind sky sometimes showed a similar effect, when large bright stars moved more than background stars as the sky’s rotation slowed.
It’s an apparent change, which means it’s not real. But by knowing how this observational trickery works, scientists can explain this effect and determine where an object really is in space. Not much can be learned from a year of observations. But Gaia is a long-haul mission, and ESA plans to track the positions of 1% of stars in the Milky Way for Gaia’s 12-year mission, monitoring about a billion galactic stars.
Over time, as scientists account for parallax and another awesome force caused by Earth’s speed through space called aberration, they notice that stars move ever so slightly in space.
What did the night sky look like in the days of the dinosaurs?
Gaia project scientist Timo Prusti said that if we turn the clock back a few thousand years, the stars in the Milky Way would appear to move in a linear fashion. But if we zoomed out on the Milky Way and followed the movement of the sun over millions of years, we would see that the sun and its planets would try to escape from the galaxy, but would then be pulled back inside, then back down the other way, oscillating up and down as they move around the center of the Milky Way to create a curved line.
Everything in our corner of the Milky Way would travel alongside us at about the same speed, Prusti said. But the sky would look quite different, and that’s again thanks to parallax. The stars closest to us have a high proper motion. Each year, they seem to move much farther along the canvas of the night sky than the much more distant stars.
If you went back far enough in time, you would have a whole different night sky. Dinosaurs from the Middle Cretaceous geologic period would have been on the far side of the Milky Way’s disk because the sun takes about 220 million years to once travel through the galactic core.
“I think the dinosaur sky was totally different,” says Prusti Reverse. “Of course, more or less everything in the neighborhood is moving at roughly the same speed. However, that’s only roughly. This means that in this time scale of a million years, the sky would be totally different.
At larger timescales, you may also notice that some stars move countercurrent to others. These are the remnants of the crash of Gaia-Enceladus-Sausage, the ancient galaxy that crashed into a proto-Milky Way 10 billion years ago.
Zodiac Blues — But, back to moon knight. Even just 2,000 years of night sky drift is enough to confuse the location of the constellations. Zodiac signs correlate with certain months because the ecliptic, the Sun’s apparent path across the sky, gives rise to constellations, all of which are unique from the solar system’s perspective. Fast forward thousands of years after this association was first noticed, and the ecliptic no longer passes through the same set of stars. Think you’re a Virgo? It’s a lie, you’re something else.